Parabolic antenna reflectors are widely used in both ground-based and space-based applications. These structures are large in size, and typically have a diameter of up to 20 meters. It is important that parabolic antenna reflectors be accurately built to avoid performance degradations that may result from thermal distortion, manufacturing tolerances, alignment tolerances, deployment tolerances or other error sources.
To determine how accurately ground-based parabolic antenna reflectors are built, interferometers are commonly used. The accuracy and precision of an interferometer is generally within fractions of its operating wavelength. Interferometers are ideal for profiling ground-based parabolic antenna reflectors because these structures are built very rigid. Rigid structures do not sway or vibrate, and consequently, can be measured with relatively high precision (for example, micron range uncertainties can be profiled).
A spaced-based parabolic antenna reflector typically includes a rib-like structure that folds up like an umbrella so that it can be loaded into a rocket and carried into outer space. Once in outer space, the rib-like structure is removed from the rocket and unfolded. Unfortunately, a space-based parabolic antenna reflector does not have the same structural integrity and stiffness as a ground-based parabolic antenna reflector. After being unfolded, the resulting shape may be distorted from an ideal desired shape, and this causes performance degradation in applications that depend on precise dimensional relationships.
In most cases, this degradation in performance can be corrected by adjusting the shape of the parabolic antenna reflector using on-board actuators, provided the distortions are accurately measured. However, a conventional interferometer can not be used to measure these distortions because the movement or sway of spaced-based parabolic antenna reflectors is much greater than the unambiguous measuring range of the interferometer.
When using an interferometer, light or dark bands are produced by the interference or diffraction of light being reflected from a parabolic antenna reflector to an optical detector. If the light and dark bands (i.e., fringes) are spaced in the micron range, yet the space-based parabolic antenna reflector being profiled is swaying in the millimeter range, this causes large uncertainties due to fringe ambiguity and fringe washout. Consequently, an interferometer can not be used for profiling the space-based parabolic antenna reflector, and other techniques must be used.
U.S. Pat. No. 6,293,027 to Elliott et al. discloses one such technique. A first set of targets on the space-based parabolic antenna reflector is scanned by an attitude transfer system to measure the angular location and range of each target relative to a reference point on another part of the satellite having a frame of reference. The orientation of the parabolic antenna reflector is then determined from the measured locations of the targets. A second set of targets on the parabolic antenna reflector is scanned by a figure sensing module located at a reference point on the reflector itself. From measured angular locations and ranges of the second set of targets, shape distortions in the parabolic antenna reflector can be determined, and distortion may be corrected. Unfortunately, since each of the first and second sets of targets is scanned, this technique may be relatively complex and slow in the sense that all of the targets are sequentially measured.